1. Field of the Invention
The present invention relates to a digital hearing aid and its hearing sense compensation processing method using digital signal processing with a sound sensing hearing impairment as an object.
2. Description of the Related Art
A hearing sense lesion, i.e., hearing impairment can be conventionally mainly divided into two kinds of a sound transmitting hearing impairment and a sound sensing hearing impairment. The sound transmitting hearing impairment is a hearing sense lesion caused by a change in sound transmitting characteristics since a certain lesion is caused in one or all of an external ear, a middle ear, a round window and an oval window. The sound transmitting hearing impairment can be overcome by simply amplifying an input sound.
In contrast to this, the sound sensing hearing impairments is a hearing sense lesion in which it is considered that there is an organic disease lesion in a portion from the middle ear to a cortical auditory area. The sound sensing hearing impairment shows a state in which it is difficult to sense a sound itself by abnormality of the middle ear, etc.
The sound sensing hearing impairment is caused since there is no stereocilia at an end tip of a hair cell of a cochlea and there are a lesion of a nerve for transmitting a voice, etc. Presbycusis is included in this sound sensing hearing impairment.
It is difficult to overcome the sound sensing hearing impairment by a hearing aid constructed by only a conventional simple amplifier. Recently, a digital hearing aid capable of performing complicated signal processing has begun to be noted. An individual difference is various and large with respect to symptoms of the sound sensing hearing impairment. There is a recruitment phenomenon of a loudness as one of the main symptoms. A sound pressure is a physical quantity of a sound and the loudness is a sound amount sensed when a human being hears a sound at a certain sound pressure, i.e., a sensing amount.
In the recruitment phenomenon, as shown in FIG. 1, an audible minimum level (a minimum hearable value, HTL) is raised and no maximum level (maximum hearable value, UCL) is changed so much and a hearable range (auditory area) is narrowed in comparison with a normal hearing person. The maximum hearable value is slightly reduced in many cases. Namely, a small sound is inaudible and a large sound can be heard at a loudness as in the normal hearing person in this phenomenon. Therefore, when the small sound is amplified to hear the small sound by a hearing aid, etc. and the large sound is inputted, an output sound exceeds a maximum hearable value so that the large sound attains an uncomfortable level and is inaudible. Therefore, it is necessary to amplify the small sound with a large gain and amplify the large sound with a small gain. One of the features of the recruitment phenomenon is also that the above changes in hearing ability are different from each other every frequency.
Countermeasures of the above sound hearing impairment are taken in the following three prior arts.
There is a technique described in Japanese Patent Application Laid Open No. 3-284000 hereinafter referred to as prior art 1. In this prior art, the dynamic range of an input sound is compressed within a narrowed hearable range of a hearing impairment person. FIGS. 2A to 2E show a hearing sense compensation processing method of a hearing aid using this method. In FIG. 2A, an axis of abscissa shows a sound pressure and an axis of ordinate shows a loudness. A curve shown by a solid line shows the relation of the sound pressure and the loudness with respect to a normal hearing person. A curve shown by a broken line shows the relation of the sound pressure and the loudness with respect to the hearing impairment person. As can be seen from FIG. 2A, when the normal hearing person and the hearing impairment person hear a sound at a certain sound pressure, the normal hearing person senses this sound as a large sound in comparison with the hearing impairment person. When the heard sound pressure is set to be smaller than that at a minimum hearable threshold value of the hearing impairment person, no hearing impairment person can hear this sound although the normal hearing person can hear this sound.
A solid line of FIG. 2B shows the relation of a sound pressure sensed as an equal loudness by the above normal hearing person and the hearing impairment person. Axes of ordinate and abscissa of FIG. 2B respectively show a sound pressure level with respect to the hearing impairment person and a sound pressure level with respect to the normal hearing person. The difference between sounds sensed as the same loudness by the hearing impairment person and the normal hearing person is increased as the sound pressure is reduced. This difference is reduced as the sound pressure is increased. Here, a broken line shows that a straight line relation at a large sound pressure level is extrapolated until a sound pressure level 0 as it is. This broken line also shows the relation of a sound pressure level provided when normal hearing persons are compared with each other. The relation of the sound pressure shown by this broken line is shown by a straight line. In FIG. 2B, when the sound pressure level with respect to the normal hearing person is considered as an input and the sound pressure level with respect to the hearing impairment person is considered as an output, the relation shown by a solid line of FIG. 2C is obtained. A broken line of FIG. 2C shows the relation of input and output levels when these input and output levels are equal to each other. When the hearing aid amplifies an input sound with the difference between solid and broken lines of FIG. 2C as a gain, the hearing impairment person can sense the input sound as a sound having the same loudness as the normal hearing person.
FIG. 2D shows the relation between a gain calculated as mentioned above and an input sound pressure. When the input sound pressure is reduced, the gain is increased. The gain is reduced as the input sound pressure is increased.
FIG. 2E is a view conceptually showing a calculating method of the gain of the hearing aid calculated from loudness curves of the normal hearing person and the hearing impairment person and an intensity (sound pressure level) of the input sound. In FIG. 2E, an axis of ordinate shows a loudness level [phon] and an axis of abscissa shows a sound pressure level [dB] of the input sound. A solid line in FIG. 2E shows a loudness curve of the normal hearing person and a one-dotted chain line shows a loudness curve of the hearing impairment person.
FIG. 2E is a graph of a loudness curve showing the loudness of an input sound heard by each of the normal hearing person and the hearing impairment person. In FIG. 2E, an axis of abscissa shows a sound pressure level (dB) and an axis of ordinate shows a loudness (phon). The axes of ordinate and abscissa of FIG. 2E are shown by logarithm. As shown in FIG. 2E, the normal hearing person hears a sound heard at a loudness c' as a sound at a sound pressure c, and the hearing impairment person hears a sound heard at the loudness c' as a sound at a sound pressure c". Namely, when the hearing impairment person hears the sound at the sound pressure c by amplifying this sound until the sound pressure c", the hearing impairment person hears the sound at the same loudness as the sound at the sound pressure c heard by the normal hearing person. The gain of the hearing aid shows that the above sound pressure c is amplified to the sound pressure c". The loudness curve shown in FIG. 2E is shown by logarithm on both the axes of ordinate and abscissa. Therefore, the gain G is calculated from the following formula 1. EQU G=c"-c (1)
Here, c" shows a sound intensity heard by the hearing impairment person and c shows the intensity of an input sound. It is known from the formula 1 that the gain is increased as the difference between c" and c is increased.
There is a thesis entitled "Consideration of a hearing impairment person hearing system by noise suppression processing and automatic gain control" hereinafter referred to as prior art 2. This thesis is described on page 415 of a lecture thesis collection of a meeting for reading research papers in Acoustic Society of Japan, in spring, 1996. FIG. 3 is a block diagram showing the construction of this hearing impairment person hearing system.
In this construction, an input sound is first linearly estimated and analyzed (LPC analyzed) in a voice/non-voice discriminating section 1 so that spectral inclusive characteristics and an estimate residual signal are obtained. Next, a correlation of this residual signal is calculated. If a peak value of this residual signal is equal to or greater than a threshold value, this signal is set to a signal in a voice section. In contrast to this, if the peak value is equal to or smaller than the threshold value, this signal is set to a signal in a non-voice section. The voice section shows a signal and the non-voice section shows a noise.
Next, FFT (Fast Fourier Transform) 3 is performed with respect to an input signal and weighting 4 is performed by a function calculated from spectrums of the non-voice section and the voice section with respect to a spectrum of a portion discriminated as a noise in a noise suppression processing section 2. The weighted spectrum is then subtracted from a spectrum of the input signal so that noise suppression processing is performed.
Next, an inverse FFT 5 is performed with respect to the noise suppression processed signal and the obtained data are sent to an automatic gain control section (AGC section) 6. A compression/extension section 7 of the automatic gain control section 6 compresses and extends this signal. In compressing and extending methods of this compression/extension section 7, a compression threshold value 9 is first updated from an executing value 8 of a portion discriminated as a non-voice. When the executing value 8 of the noise suppression processed input signal is equal to or greater than the threshold value 9, the input signal is compressed. In contrast to this, when the executing value 8 is equal to or smaller than the threshold value, the input signal is extended. Thus, emphasis of a residual noise left in erasure of the noise suppression processing section 2 is prevented.
An average value 10 of the executing value equal to or greater than the threshold value for past several seconds is calculated to make a gentle gain adjustment and the compression/extension section 7 performs the compression and extension processes with respect to this average value 10. The automatic gain control section 6 multiplies a compression extension rate and a gain 11 by an input frame provided after the noise suppression processing, and outputs the multiplied results.
There is a thesis entitled "Development of multi-signal processing type digital hearing aid" hereinafter referred to as prior art 3. This thesis is described on pages 519 and 520 of a lecture thesis collection of a meeting for reading research papers in Acoustic Society of Japan, in autumn, 1994. FIGS. 4A and 4B show a dynamic range compressing method used in this thesis. In FIG. 4A, an axis of abscissa shows a sound pressure level of an input signal, and an axis of ordinate shows a sound pressure level of an output signal. In FIG. 4A, parameters on the axes of ordinate and abscissa in FIG. 2B are changed and are shown in a unit HL. HL is a unit with respect to a hearing ability level and shows the difference in level between a reference minimum hearable value and an output sound pressure within a prescribed coupler of an earphone for an audiometer at a certain frequency. Here, an intermediate hearable value is an intermediate value between lower and upper limit levels judged as "just good" by a tested person. Here, two kinds of dynamic range compressing methods are used.
One of the dynamic range compressing methods is a loudness compensating method in which a voice band is divided into 3ch and a nonlinear amplifying operation is performed in conformity with hearing ability characteristics of the hearing impairment person. Namely, the loudness compensating method is a method for compressing a dynamic range of the normal hearing person to a dynamic range of the hearing impairment person. This method is shown by a solid line in the graph of input and output sound pressure levels in FIG. 4A.
The other of the dynamic range compressing methods is a voice dynamic range mapping method in which the dynamic range is compressed such that 20 dBHL corresponds to a minimum hearable value of the hearing impairment person. This method is shown by a broken line in the graph of input and output sound pressure levels in FIG. 4A.
This method is shown by the graph of FIG. 4B showing the relation of the sound pressure and the loudness. As can be seen from FIG. 4B, the inclination of a straight line approximate to a loudness curve of the normal hearing person is changed.
However, these prior arts have the following defects. Namely, in the case of the prior art 1, the gain with respect to an input sound is increased as a sound pressure level is reduced. As a result, a circumferential small noise not to be originally heard is amplified with a very large gain. Accordingly, the input sound obtained by hearing sense compensation processing includes the noise amplified with a very large gain in a non-voice portion. Therefore, it is difficult for a user to hear a subsequent voice by masking in a time direction.
In the case of the prior art 2, no hearing ability characteristics of the hearing impairment person greatly different from each other every individual are considered. As a result, there is a case in which the gain of a high sound portion is too small and the gain of a low sound portion is too large with respect to a person having low hearing ability in a high sound. As a result, no sound can be heard in the high sound portion by insufficient amplification and the gain exceeds a maximum hearable value in the low sound portion so that no sound can be heard. A reverse phenomenon can be caused with respect to a person having low hearing ability in a low sound.
In the case of the prior art 3, no input sound equal to or smaller than 20 dBHL is amplified and only an input sound equal to or greater than 20 dBHL is amplified in conformity with a loudness of the input sound. Therefore, a gain with respect to the input sound slightly exceeding 20 dBHL becomes maximum. As a result, the input sound slightly exceeding 20 dBHL is amplified with a very large gain so that an output sound becomes a sound brokenly heard and having large noises and difficult to be heard.